1. Field of the Invention
The present invention relates to mobile telecommunication systems. In particular, the present invention relates to a novel and improved method, system and protocol entity for sending packet data units for unacknowledged mode services.
2. Description of the Related Art
In wireless data communications base stations are used to transmit data to mobile terminals. Mobile terminals can be in connection with one or more base stations at a time. In other words, cells of base stations may overlap, i.e. in some point a mobile terminal may be within two or more cells of different base stations. Every base station is able to serve mobile stations within a certain range. If a mobile terminal drifts out of the range of a base station, service is not available any more. Therefore, a handover to a new base station is needed. Normally, mobile telecommunication networks consist of several cells constituting a larger service area.
Most mobile telecommunication systems comprise a feature called as a ‘handover’. A handover procedure means that an already established connection (e.g. a call, real-time data connection, non real-time data connection etc.) to a mobile terminal is maintained while moving from a cell to cell within a mobile telecommunication network. Examples of mobile communication networks include Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications (GSM), Internet Protocol Radio Access Network (IP RAN), Wireless Local Area Network (WLAN), etc.
In general, handovers are divided into two categories: a hard handover and a soft handover. A hard handover means that all the old radio links in the user equipment are removed before new radio links are established. A soft handover means that the radio links are added and removed in a way that the user equipment always keeps at least one radio link to a radio access network. A soft handover is performed by means of macro diversity, which refers to the condition that several radio links are active at the same time. Normally a soft handover can be used when cells operated on the same frequency are changed.
The handover procedure for data connections can be problematic. In a WCDMA network with the High Speed Downlink Packet Access (HSDPA), packet scheduling is done in a Node B. A buffer with user data is located in Node Bs on top of the Radio Link Control (RLC) buffer with user data in a Radio Network Controller (RNC). The data flow between these two buffers is regulated by flow control. The handover decision is taken in the RNC. When a handover between cells belonging to a different Node B is triggered, the RNC has to know the starting point of the data transmission to the new Node B.
The handover situation is problematic e.g. in the HSDPA. One of the key features of the HSDPA, the Packet Scheduler, is located in a Node B with the purpose of using recent channel quality information when scheduling users in a cell. That implies that user data is transferred from a radio network controller to the Node B, and queued in the Node B waiting to be transmitted from the Node B to user equipment. In the case of some services, e.g. streaming services, the amount of data stored in the Node B may be significant, and can easily reach values up to 20-30 Kbytes or even larger. Under the specifications of the 3GPP, the radio network controller is able acquire information about the status of the data transferred through the Iub interface when acknowledged mode transmission is used, i.e. whether data have been successfully received by the user equipment or if data is still stored in the Node B. However, when unacknowledged mode transmission is used, the radio network controller does not have the status of the data transferred through the Iub interface.
On the other hand, the HSDPA does not support a soft handover. So, the handover can be done in e.g. two ways: 1) a HS-DSCH (High Speed Downlink Shared Channel) hard handover, 2) via mapping the bearer into a dedicated channel (DCH), and activating a soft handover in the cell edge area, and then return the bearer mapping into the HS-DSCH as soon as the user equipment enters the HSDPA cell coverage area (as shown in FIG. 1).
For services using unacknowledged mode in both handover options, the radio network controller faces the difficulty of not having information of which data packets have been successfully transmitted to the user equipment or are still stored in the Node B. Thus, when the process of a handover takes place, the radio network controller cannot identify which packets of the data flow have already been transmitted from the initial Node B to the user equipment, and which packets of the data flow should be transmitted from the new Node B to the user equipment.